Superelastic NiTi polycrystalline shape memory alloy undergoes deformation instability when subjected to uni-axial stretching, and transforms from initial austenite to a high-strain phase, i.e. martensite, by the nucleation and growth of macroscopic domains as deformation processes. This stress-induced first-order phase transformation involves heterogeneous localized release and transfer of latent heat, which in turn influences the mechanical responses and deformation processes due to the material’s intrinsic mechanical nonlinearity and thermo-mechanical coupling. The macroscopic martensitic domain exhibits different patterns in different geometry configurations of shape memory alloys. It was observed that parallelogram-shaped domains nucleate in quasi-1D strip geometry while helix-shap...[ Read more ]

Superelastic NiTi polycrystalline shape memory alloy undergoes deformation instability when subjected to uni-axial stretching, and transforms from initial austenite to a high-strain phase, i.e. martensite, by the nucleation and growth of macroscopic domains as deformation processes. This stress-induced first-order phase transformation involves heterogeneous localized release and transfer of latent heat, which in turn influences the mechanical responses and deformation processes due to the material’s intrinsic mechanical nonlinearity and thermo-mechanical coupling. The macroscopic martensitic domain exhibits different patterns in different geometry configurations of shape memory alloys. It was observed that parallelogram-shaped domains nucleate in quasi-1D strip geometry while helix-shaped domains nucleate in quasi-2D thin-walled tube geometry. In this thesis, effects of length scales and time scales on the macroscopic domain pattern selection are experimentally investigated and theoretically analyzed.

For quasi-1D strip configuration, effects of time scales on macroscopic domain pattern selection are addressed in terms of rate-dependence and ambient-dependence of parallelogram-shaped martensitic domain patterns in NiTi strips, respectively under a wide loading rate range from stretching rates (10^-4/s~10/s) by standard hydraulic testing machine to intermediate stretching rates (10¹/s~10²/s) by custom-modified Split Hopkinson Tension Bar (SHTB), and in four different ambient convection conditions including still air, still water, 2m/s and 4m/s flowing water. For quasi-2D tube configuration, systematic experimental investigation and theoretical analysis are conducted to investigate the effects of length scales on equilibrium helical domain pattern in different tube geometries (length L, wall-thickness h and tube radius R) under isothermal stretching, and rate-dependence (loading time scale) of helical domain pattern evolution in one tube under stretching rates 10^-4/s~10^-1/s. The processes of macroscopic domain pattern evolution are recorded by in-situ optical camera equipped with zoom lens at different recording rates. The local temperature variations are measured by very fine thermocouples of 25μm wire-diameter. The evolution of macroscopic martensitic domain patterns, local temperature variations, and the corresponding stress-strain responses are quantitatively synchronized both for strips and tubes.

It is found that the equilibrium martensitic helical domain patterns in tube geometry under isothermal stretching are governed by two distinct strain-misfit energy terms which depend on length scales (tube geometry). It is revealed that non-isothermal martensitic domain patterns both in strip geometry and tube geometry are essentially governed by the competition between two groups of time scales: one loading time scale (transformation time) which governs release rate of latent heat, two heat-transfer time scales including characteristic conduction time and characteristic convection time which govern transfer rate of latent heat. Power-law scaling relations are extracted both for effects of length scales and time scales on macroscopic domain patterns. There are good agreements between the experiments and theoretical rationales.

Keywords: shape memory alloy; martensitic phase transformation; macroscopic domain pattern; length scales; time scales.